How do they know that any individual cancer is caused by any particular agent, down to a tenth of a percent?

Ice - Nothing is revealed about this particular agent's identity or backstory.

The Shadow had trained this particular agent to be more than thorough when examining such scenes.

When used in clinical practice, the particular agent used may vary based on the degree of response required.

This particular agent also has the advantage of wearing off more quickly than any sedative.

No single product completely satisfies these requirements so the use of any particular agent is always a compromise.

With that particular agent, a single drop on the skin would kill.

One particular agent is peddling an ugly piece of information.

Among the millions of lymphocytes in the human body, only a few actually are specific for any particular infectious agent.

This particular agent is an expert in bringing a man to his knees.